Hm, this is the opposite of what I'd expect. "Known" is to indicate
that a field may exist in a file, whereas the standard is to indicate
it can be derived. By that logic, "Known" is where "density" should
go and standard is where "Density" should go, which in my version of
the code is how it is done.

One quick thought. Can you try with the validators for 'density'
removed, using the standard code as it is in the tip of the main repo?
i.e., change